FIG. 3 is a schematic sectional view illustrating a conventional method for fabricating a device using a semiconductor film. In FIG. 3, reference numeral 1 designates a semiconductor film, reference numeral 3 designates a substrate, reference numeral 10 designates an isolation film, reference numeral 11 designates a encapsulation film. The semiconductor film 1 comprises, for example polycrystalline silicon or amorphous silicon, which is formed by a method such as thermal decomposition or plasma decomposition of gases containing silicon, such as silane or dichlorosilane. The substrate 3 is formed of the same material as that of the semiconductor film 1 or a material whose melting point or softening temperature is higher than the melting point of the semiconductor film 1. For example, as the material of the substrate 3, a crystalline silicon wafer, sapphire, quartz, a ceramic or the like is used. When the semiconductor film 1 is silicon and the substrate 3 is also silicon, the isolation film 10 is inserted between the semiconductor film 1 and the substrate 3 in order to electrically insulate and to structurally and thermally isolate the semiconductor film 1 from the substrate 3. In most cases, silicon dioxide is used as the isolation film 10. Even when the substrate 3 is formed of a material different from that of the semiconductor film 1, if a material with high thermal conductivity such as a ceramic is used, an isolation film 10 with low thermal conductivity such as silicon dioxide film is generally inserted between them to thermally isolate the semiconductor film 1 from the substrate 3. In addition, the isolation film 10 prevents a component of the substrate 3 or an impurity contained in the substrate 3 from diffusing into the semiconductor film 1 so as not to degrade the performance of the semiconductor device. The silicon dioxide film is also generally used as the encapsulation film 11 like the isolation film 10. The encapsulation film 11 protects the surface of the semiconductor film 1 from the atmosphere and mechanically maintains the morphology of the semiconductor film 1.
Next, operation thereof will be described. The whole sample with the above structure is heated up at a temperature close to the melting point of the semiconductor film 1, which is approximately 1200.degree. to 1300.degree. C. if the semiconductor film 1 is silicon because the melting point of silicon is 1414.degree. C. Although the heating means is not shown in FIG. 3, heating with an infrared lamp heater or a carbon heater, radio frequency induction heating or the like is used in most cases. In this state, a part of the sample is further heated to melt a part of the semiconductor film 1. As the partial heating means, an infrared lamp heater, carbon heater or the like is also generally used. Then, the partially heated region is moved to sequentially melt the semiconductor film 1 from one end of the sample and then it is solidified, that is, recrystallized again from the rear region of the partially heated part. When the semiconductor film 1 solidifies, since solidification following the crystalline orientation of the rear part which has been already recrystallized, the recrystallized seniconductor film 1 consists of large grains in which the crystalline orientation is uniform regardless of crystallinity of the semiconductor film 1 before being melted, even if the semiconductor film 1 before being melted is noncrystalline. In addition, a monocrystalline semiconductor film 1 can be obtained by bringing a part of the semiconductor film 1 into contact with a semiconductor monocrystal formed of the same material as that of the semiconductor film 1 and then solidifying it so as to follow the crystalline orientation of the semiconductor monocrystal when the semiconductor film 1 is recrystallized.
Since the semiconductor film 1 is sandwiched between the isolation film 10 and the encapsulation film 11, thermal energy applied to the semiconductor film 1 by the partially heating means is confined in the semiconductor film 1. Therefore, the thermal energy applied to the semiconductor film 1 can be effectively used to melt it and thus the substrate 3 is prevented from being superheated. In addition, when the semiconductor film 1 is melted and becomes liquid, since the encapsulation film 11 covers the semiconductor film 1, the semiconductor film 1 is prevented from agglomerating like dew because of surface tension and the configuration of the film 1 is prevented from changing during the recrystallization. When the semiconductor film 1 is silicon, silicon dioxide film is generally used as the encapsulation film 11. However, since the interface energy between the melted silicon and the silicon dioxide film is large and the silicon dioxide film is softened at the melting temperature of silicon, the configuration of the semiconductor film 1 can not be effectively prevented from changing only by a silicon dioxide film in some cases. In this case, the encapsulation film 11 is a silicon nitride film laminated on a silicon dioxide film. By using the silicon nitride film, wetting between melted silicon and the encapsulation film 11 is improved and the forces which cause agglomeration are reduced. In addition, the mechanical strength of the encapsulation film 11 is reinforced, whereby ability to prevent the configuration of the semiconductor film 1 from changing is enhanced by a silicon nitride film.
However, according to the conventional method for fabricating the semiconductor device, since the material whose melting point or softening temperature is as same as or higher than the melting point of the semiconductor film 1 is used for the substrate 3, the substrate 3 is in a solid state when the semiconductor film 1 is melted and recrystallized. Therefore, thermal stress stored in the semiconductor film 1 remains in a crystalline semiconductor film 1 when the semiconductor film 1 is solidified, which influences the characteristics of the semiconductor device fabricated using the semiconductor film 1. More specifically, when the material of the substrate 3 is different from that of the semiconductor film 1, because of the difference of the thermal expansion coefficients between the semiconductor film 1 and the substrate 3, stress corresponding to the distortion of (.alpha.s-.alpha.b).times.(Tm, s-Tr) remains in the semiconductor film 1, where Tm, s is the temperature at which the semiconductor film 1 is melted and recrystallized, .alpha.s is an average thermal expansion coefficient of the semiconductor film 1, .alpha.b is an average thermal expansion coefficient of the substrate 3 and Tr is the room temperature.
Even when the substrate 3 is formed of the same material as that of the semiconductor film 1, since the semiconductor film 1 is usually thermally isolated from the substrate 3 by the isolation film 10, the temperature Ta of the substrate 3 is lower than the melting point Tm, s of the semiconductor film 1 at the moment the semiconductor film 1 is melted and solidified. Therefore, in this case also, stress corresponding to the distortion of .alpha.s.times.(Tm, s-Ta) remains in the semiconductor film 1. In addition, if the semiconductor film 1 is silicon, when silicon is solidified, its volume is expanded from that in a melted state by 9%. Therefore, when silicon is solidified on the solid substrate 3 or the isolation film 10 attached thereon, stress corresponding to volume expansion at the time of solidification is inevitable.
FIG. 4 is a view showing a method for manufacturing a semiconductor device disclosed in Japanese Published Patent Application No. 63-88819. In FIG. 4, reference numeral 100 designates an insulating film such as a silicon dioxide film or a silicon nitride film disposed on a surface of the substrate 3, reference numeral 22 designates a thin film of Ge or the like with the melting point lower than that of the semiconductor film 1 which is silicon, and reference numerals 111 and 112 designate films disposed on a part of the encapsulation film 11 in order to reduce the power of the energy beam applied to the semiconductor film 1. In this example, a thin film 22 with a thickness of 400 to 800 nm whose melting point is lower than that of the semiconductor film 1 is formed between the semiconductor film 1 and the substrate 3. When the semiconductor film 1 is partially heated melted and recrystallized by a heating means such as an electron beam, the semiconductor film 1 is solidified on the thin film 22 by melting the semiconductor film 1 and the thin film 22 simultaneously, with the result that stress caused by thermal expansion is prevented from being generated in the semiconductor film 1. More specifically, the thin film 22 is melted between the temperature Tm, s at which the semiconductor film 1 is recrystallized and the temperature Tm, b, where Tm, b is the melting point of the thin film 22. Therefore, stress caused by a change of volume at the time of solidification of the semiconductor film 1 is relieved and stress caused by thermal expansion of the semiconductor film 1 at the temperature from Tm, s to Tm, b is not generated, so that the stress remaining in the semiconductor film 1 is reduced to that corresponding to the distortion of (s-b).times.(Tm, b-Tr) (Tm, b&lt;Tm, s).
However, in this conventional example, since the thin film 22 with the low melting point is melted by heating with the energy beam for the melting of the semiconductor film 1, the melted part of the thin film 22 is limited to the vicinity of the melted part of the semiconductor film 1 and the thin film 22 is also melted and solidified as the semiconductor film 1 is melted and solidified. Accordingly, thermal strain in the thin film 22 itself is generated, which applies stress to the semiconductor film 1.
This arrangement has been devised mainly with a view to forming the thin film crystalline silicon on the insulating film 100. Such a structure is called SOI (Silicon On Insulator), which has been actively studied as a fundamental structure for implementing a three dimensional integrated circuit, that is, a device with the structure in which plane integrated circuits are stacked by forming a crystalline silicon thin film on a surface of the silicon substrate 3 with the integrated circuit thereon and further forming the integrated circuit thereon and then connecting the respective integrated circuits to each other by wiring. However, in this case, the thin film 22 formed of a material with the low melting point such as Ge is disposed between the substrate 3 and the semiconductor film 1 and the thin film 22 still remains between them after the semiconductor film 1 is recrystallized. Therefore, when the three dimensional integrated circuit is implemented, it is necessary to wire the semiconductor film 1 to the substrate 3 through the thin film 22 between them. However, when a conductive material such as Ge is used for the thin film 22, since the wirings is short-circuited, the semiconductor film 1 can not be properly electrically connected to the substrate 3, so that it is difficult to implement the three dimensional integrated circuit. In addition, since the melting point of the thin film 22 is low, the thin film 22 is melted during the fabrication process for forming the integrated circuit on the semiconductor film 1 at a temperature above the melting point of the thin film 22. Thus, the force holding the semiconductor film 1 on the substrate 3 is lost and the position of the semiconductor film 1 on the substrate 3 could be shifted. As a result, it becomes difficult to form wiring correctly between the semiconductor film 1 and the substrate 3, so that it is difficult to form the three dimensional integrated circuit in this case also. As described above, although the conventional example shown above is effective in principle as far as recrystallization of the semiconductor film 1 is concerned, in the actual application as fabrication technique, there are many problems to be solved.
In addition, since the semiconductor film 1 is disposed on the substrate 3 in the above conventional examples, the substrate 3 is distorted because of the stress generated after melting and recrystallization of the semiconductor film 1 and then the planarity of the semiconductor film 1 is damaged. In addition, since the melting and recrystallization of the semiconductor film 1 is performed on substrate 3, the size of the semiconductor film 1 is limited to the size of the substrate and a semiconductor film 1 having a large area can not be recrystallized.
As described above, according to the conventional method for fabricating the semiconductor device, stress is generated in the semiconductor film by its melting and recrystallization, whereby the substrate is distorted and the planarity of the semiconductor film can not be maintained. Even when generation of the stress can be reduced, it is difficult to actually form the semiconductor device by the above method. In addition, the size of the semiconductor film is limited to the size of the substrate, so that a large area semiconductor film can not be recrystallized.